The Ear and Hearing

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The Ear and Hearing
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What are the functions of the ear?

• Hearing
• Balance

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Sensory receptors

• Sensory receptors in the ear are called
  mechanoreceptors.
• Mechanoreceptors respond to physical forces.
• Hearing receptors are stimulated by sound
  vibrations moving through fluid.
• Balance receptors are stimulated by fluid
  movement in the balance organs.

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Anatomy of the Ear

• Three major areas outer (external), middle, and
  inner (internal)
• The outer and middle ear are only used for
  hearing.
• The inner ear is used for both hearing and
  balance.

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The Outer (External) Ear

• The pinna is the shell-shaped part of the ear
  that we usually see.
• The external auditory canal is a short chamber in
  the temporal bone of the skull. This canal ends
  at the tympanic membrane.
• The tympanic membrane (eardrum) separates the
  outer ear from the inner ear.

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Middle Ear (Tympanic cavity)

• An air filled cavity in the temporal bone.
• Runs from the eardrum to a bony wall.
• The bony wall has two openings called the oval
  window and the round window.
• The auditory (Eustachian) tube connects the
  middle ear to the throat. It allows for pressure
  to be equal on both sides of the eardrum. If the
  pressures are not equal, hearing is difficult and
  the ear may hurt.

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Middle Ear cont.

• Three bones in the middle ear.
• Malleus (hammer)
• Incus (anvil)
• Stapes (stirrup)
• When the eardrum moves, the movement is
  transferred to the malleus. The malleus moves
  the incus, the incus moves the stapes. The
  stapes presses on the oval window.

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Inner (Internal) Ear

• Made of chambers called the bony labyrinth.
• Filled with a fluid called perilymph.
• Three subdivisions cochlea, vestibule,
  semicircular canals.

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How do we hear?

• Sound waves strike the eardrum.
• The eardrum transfers movement through the
  ossicles.
• The stapes transfers the movement to the oval
  window. The movement of the ossicles amplifies
  the signal.
• The movement of the oval window is transferred
  through the fluid of the inner ear.

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How do we hear? cont.

• As the fluid in the cochlea moves, it pushes and
  pulls on hair cells. This movement of hairs is
  translated into a nerve impulse.

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How does equilibrium work?

• Some receptors in the inner ear are responsible
  for static equilibrium, and some are responsible
  for dynamic equilibrium.
• Static equilibrium tells our brain which way is
  up and down.
• Receptors in the vestibule respond to how the
  head is oriented in relation to gravity.
  Information is received by hair cells and
  converted into a nerve impulse.

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How does equilibrium work? cont.

• Dynamic equilibrium tells our brain if our head
  is rotating or at an angle.
• Receptors in the semicircular canals are
  orientated in the three planes of space.
• Movement of the head causes movement of the
  endolymph. This movement is received by hair
  cells and converted into a nerve impulse.

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